The invention relates to an absorbent product comprising a liquid-permeable casing layer, a liquid-barrier layer and an absorption element which is enclosed between the liquid-permeable casing layer and the liquid-barrier layer, and a liquid-transfer layer which is disposed between the liquid-permeable casing layer and the absorption element, the casing layer being bonded in a pattern of distinct bonds to the liquid-transfer layer and both the liquid-permeable casing layer and the liquid-transfer layer comprising at least one thermoplastic component and the bonds between the envelope layer and the liquid-transfer layer being thermally produced.
A liquid-permeable surface layer for an absorbent product such as a nappy, an incontinence pad, a sanitary towel or the like must rapidly admit liquid to an absorption element disposed inside the surface layer. It is additionally highly desirable that the surface layer should feel dry against the skin of the user after the product has absorbed liquid and further that liquid which has passed into the absorption element should be prevented from running back out of the product.
In order to produce a surface layer having high liquid admissibility, high surface dryness following wetting and the capacity to prevent re-wetting, it has been proposed to use surface layers which comprise thermally bonded laminates of an upper liquid-receiving layer and a lower liquid-transfer and insulating layer. Such surface-layer laminates are described in SE 9801038-2, EP 0,685,214 and WO 97/02133.
A problem associated with the previously known surface-layer laminates has proved to be however that it has not been possible simultaneously to achieve good weldability in the integral material layers, high surface dryness in the laminate and good liquid-transferability between the surface-layer laminate and an absorption element connected thereto.
The reason why the liquid-transferability between the surface layer and the absorption element has been lower than expected is that when the material layers are bonded together in the lamination process the material is squeezed together in the bonding regions, thereby forming compressed bonding regions with cushion-like, unbonded regions in between. When such a surface-layer laminate is placed on an absorption element, only the cushion-like regions between the bonding regions will be in direct contact with the absorption element, whilst gaps are formed between the surface-layer laminate and the absorption element at the bonding points. This means that liquid transfer in and immediately around the bonding regions is lower than within those cushion-like sections of the surface-layer laminate which are in direct contact with the absorption element. There is therefore a considerable risk that liquid which meets the surface-layer laminate will not be passed on to the absorption element but instead remains in the fibre structure in and around the bonding regions. Even if liquid is transferred between the surface-layer laminate and the absorption element, the transfer at the bonding locations is effected so slowly that the surface-layer laminate stays wet for a long time and in the worst case have not been able to be drained of liquid before the next quantity of liquid meets the absorbent product.
One object of the invention is to make available an absorbent product having a surface layer consisting of a laminate which has high liquid-permeability, high surface dryness and a good capacity to resist re-wetting.
A further object of the invention is to offer a surface-layer laminate comprising a liquid-transfer layer having improved weldability.
A product realized according to the invention and of the type mentioned in the introduction is primarily characterized in that the liquid-transfer layer comprises 40-65% by weight of network-creating function fibres which are substantially intact following the bonding-together of the casing layer and the liquid-transfer layer and which have a fibre coarseness ranging from 6 denier to 12 denier and 35-60% by weight thermoplastic fibres comprising a weldable component and having a fibre coarseness of at least 3 denier.
According to one embodiment of the invention, the thermoplastic fibres comprise two different types of fibres in which the first fibre type provides weldability and accounts for 25-35% by weight of the liquid-transfer layer and the second fibre type is a bonding fibre and accounts for 25-35% by weight of the liquid-transfer layer. The first fibre type has a fibre coarseness of at least 3 denier and the second fibre type has a fibre coarseness of at least 4 denier.
The fibres in the liquid-transfer layer can be in the form of an essentially homogeneous mixture.
Alternatively, the weldable thermoplastic fibres can be substantially arranged as a part-layer forming part of the liquid-transfer layer. In this case the liquid-transfer layer comprises a first part-layer which comprises the network-creating fibres and a second part-layer which comprises the weldable thermoplastic fibres.
The network-creating fibres can be hollow fibres, solid fibres or spiral fibres. If spiral fibres are used, they should be chosen such that the degree of spiralling is relatively low. If the degree of spiralling is too high, there is namely a risk that too many fine capillaries will be formed in the liquid-transfer layer, which has an adverse effect upon the dryness of the layer and should therefore be avoided.
The thermoplastic fibres forming part of the liquid-transfer layer can comprise bicomponent fibres.
It has been shown that the composition of the liquid-transfer layer is critical to the bonding result and the functioning of the finished surface-layer laminate. One difficulty is to produce a weldable liquid-transfer layer which offers high surface dryness when it forms part, as a component, of a surface-layer laminate. In this context, it has been shown to be of great importance that the presence of thin fibres should be low in the cushion-like areas between the bonds. Thin fibres tend to produce a fine-capillary fibre structure which retains liquid instead of passing it on through the liquid-transfer layer to an inner absorbent structure.
It is therefore important that the liquid-transfer layer should contain sufficient quantity of weldable fibres to enable thermal bonding-together with an casing layer. It is additionally important that the proportion of thin fibres should not be so high that liquid remains in the fibre structure and makes the surface-layer laminate feel soggy and uncomfortable after wetting.
The fibres in the liquid-transfer layer have to fulfil three main functions. The liquid-transfer layer must therefore contain:
1) Function fibres, which create a network which ensures that the surface-layer laminate can admit liquid. The function fibres remain substantially unaffected by the production process and are therefore essentially intact even after welding, gluing and compression. The function fibres are resilient fibres which lend a certain thickness and volume to the wadding layer and counter act the formation of fine capillaries. Suitable function fibres can be hollow fibres, spiral fibres or solid fibres. If spiral fibres are used, it is important that these should not be so heavily spiralled that fine capillaries are formed. The fibres should have a coarseness ranging from 6 denier to 12 denier. Suitable materials are polyester or other fibres which are not weldable at the temperatures which are used to weld together the liquid-transfer layer with a liquid-permeable casing layer. The function fibres must also withstand, for example, thermal bonding of a fibre gauze during production of the liquid-transfer layer before this is welded together with the liquid-permeable casing layer. This means that either the fibres are not thermoplastic or they have a melting point above the temperature which is used in thermobonding and welding.
2) Welding fibres, which are used to bond together the fibre wadding with an casing layer. The welding fibres must give well-defined, distinct weld bondings. As welding fibres can be used, for example, bicomponent fibres having a polypropylene outer envelope and a polyester core and having a fibre coarseness of 3 denier.
3) Bonding fibres, which are used to bond a fibre gauze thermally to an associated wadding. The bonding fibres can be the same type of fibres as the welding fibres. The bonding fibres thus expediently consist of thermoplastic material or are made up of bicomponent fibres in which a superficially located component in the fibre is thermoplastic. The bonding fibres can, for example, be fibres having a co-polyester sheath and a polyester core, in which the sheath has a lower melting point than the core. Should the wadding be bonded by weeding, then there is of course no need for bonding fibres.
From the functional aspect, there have proved to be certain advantages associated with choosing the same type of fibres both as welding fibres and bonding fibres. Firstly, it is simpler to produce a wadding having only two components, secondly, it has been shown that a dual-component wadding feels drier than a triple-component wadding. This is probably due to the fact that more fine capillaries are formed in a triple-component system that in a dual-component system. A fibre which has been shown to function well both as a bonding fibre and as a welding fibre is a bicomponent fibre having a polyethylene outer sheath and a polypropylene core and having a fibre coarseness of 3 denier.
One difficulty has however been to produce fibres which function both as welding fibres and bonding fibres. For this reason, the functions can be separated into bonding and welding, resulting in a triple-component wadding.
In order to increase the dryness of the wadding when it is used as a liquid-transfer layer in a surface-layer laminate, it is expedient in a triple-component system to use relatively coarse bonding fibres and/or welding fibres. It has here been shown that a fibre coarseness of from 4 to 6 denier gives good results with regard to dryness.
Another way of achieving good weldability without the need to resort to a triple-component system is to arrange the welding fibres in a separate welding layer. A part-layer comprising function fibres and possibly also bonding fibres is in this case laminated to a part-layer essentially consisting only of welding fibres. As welding layer, a non-woven layer consisting holly or predominantly of polypropylene fibres can herein be used.
Sensory Determination of Surface Dryness
Surface dryness is a subjective property of a surface material. Quantitative measurements, for example of re-wetting and residual liquid in a surface material, therefore provide no reliable information on how the material will be perceived during use.
In order to decide whether a surface material feels wet, the material can be tested on a sensory basis by a test panel. Such a test panel consists of at least 10 test subjects.
In order to avoid disturbances, it is important that the tests should be carried out in a peaceful and harmonic environment with as few external stimuli as possible.
The test subjects are presented with two samples at a time and are given the task of judging the samples by simultaneously feeling both samples and indicating which sample feels wettest or coldest,
A comparison can be made of samples having the same surface material but to which different quantities of liquid have been added, as well as of samples to which liquid has been delivered two or more times. It is also possible to compare the dryness for different types of surface material. The samples in each study are ranked by comparing the number of times a certain sample has been judged to be wetter than another sample.